Chromatography is a method by which components of a solution phase are separated by the different affinities exhibited by the components for a stationary phase. Chromatography has become an important tool for measuring the compositions of materials used in the chemical, pharmaceutical, biotechnological, and food industries.
HPLC instrument is comprised of a pump for delivering and moving solvents (the mobile phase), an injector to introduce a sample of interest into the flow of the mobile phase, a tubular column encasement, containing a packed material or bed (the "stationary phase"), and a detector to register the presence and amount of different components in the mobile phase. When the mobile phase is passed through the stationary phase, each component will emerge from the column at a different time because different component in the sample will have different affinities for the packing material. The presence of a particular component in the mobile phase exiting the column can be detected by measuring changes in physical or chemical properties of the eluent. By plotting the detector's signal over time, response "peaks" corresponding to the presence of each of the components of the sample can be observed and recorded.
The resolution between response peaks in a chromatographic analysis or "run" depends, in part, on providing a uniform and reproducible flow of the mobile phase through the stationary phase. Irregularities or changes in the packing material in the column from run to run adversely affect reproducibility of runs, and the reliability of the chromatographic analysis. For example, voids in the packed bed create flow irregularities, leading to overlapping responses or muted response peaks.
The chromatography column encasement is typically stainless steel. This encasement is tightly packed with the stationary phase material by slurry packing, by tapping, or by mechanical ramming.
Within rigid-wall steel columns, voids can occur not only within the packing, but also at the interface between the packing and the encasement wall. This leads to a phenomenon referred to as side or wall channeling, where the mobile phase travels down the wall of the column effectively bypassing the packing or stationary phase. Such side channeling decreases the reliability and reproducibility of the chromatographic peaks, and hence their analytical value.
While a tightly packed bed is less prone to deterioration, degradation still occurs no matter how well the packing is initially done. Vibration during shipping and handling, temperature fluctuations, and/or mobile phase changes can also cause the formation of voids.
Columns have been made with a plastic encasement containing the packing materials. However, such plastic columns tend to have poor performance. Under the pressure of a mobile phase, the plastic expands and voids are formed between the packing material and the plastic tube wall resulting in the loss of performance.
A typical plastic column is packed such that the chromatographic medium is under initial radial compression. This radial compression is provided by the plastic walls of the column. When used, the column is further radially compressed under the influence of gas or liquid. To achieve this the column is placed in a pressure vessel and subjected to external compression pressures up to 1000 psi. This approach requires the chromatography bench to be equipped with an appropriately sized apparatus to provide compression of the column. Such apparatus adds to the expense of the standard chromatography work station.
A simple mechanism to impart and maintain radial compression on chromatography columns is desired.